Proper food preservation is an important activity because in addition to providing palatable and nutritional products it protects people and animals for food-born diseases, especially bacteria, fungi, and parasites. It also prevents the production of toxins after harvest and provides for extended usage of food resources. Drying, salting and smoking are ancient methods of preserving food. Canning is one of the major methods which allows extended shelf life of foods. Sterilization with heat in plastic packaging is a more recent successful method. Newer promising methods include radiation, and even high magnetic fields can reduce the bacterial counts for some organisms. Freezing is an expensive method of preserving, transporting, marketing and storing foods and the cellular destruction of food which occurs when foods are frozen is undesirable for some foods.
It was discovered early in this century that only a limited number of foods could be sterilized by pressure alone (Hite, et al., 1914) Peaches and pears were preserved and stored for 5 years, after treatment of about 400 MPa (57,000 psi). Apple juice with natural flora treated at pressures above 400 MPa (57,000 psi) for 1/2 to 2 hours did not ferment. By contrast fermentation of blackberries and raspberries usually occurred after similar treatment. Very few samples of tomatoes subjected to 680 Mpa (97,000 psi) for 1 hour were sterilized. Every sample of peas, beans, beets or other vegetables spoiled after such treatment. Because of this work, for many years little attention was paid to this potential method of food preservation.
Recently orange juice, kiwi jam, and strawberry jam were successfully preserved by pressure treatment (Hayashi, 1990).
The pressure treatment of some micro-organisms to pressures of 63 MPa (600 atmospheres) for 2 days caused growth cessation. These species included, Alcaligenes viscosus, Bacillus subtilis, Escherichia coli, Micrococcus luteus, Mycobacterium phlei, Proteus vulgarius, Pseudomonas fluorescens, Sarcina lutea, Serriatia marcescens, Staphylococcus aureus, Streptococcus lactis, Hansenula anaomala, Saccharomyces cerevisiae and Torula cremoris (ZoBell and Johnson. 1949). For shorter periods and higher pressures sterility is not obtained for many bacterial species. Salmonella recover to 0.1% of initial number in 4 hours after a 30 minute exposure to 510 MPa (5,000 atmospheres, 73,000 psi). Saccharomyces cerevisiae are killed in apple juice, orange juice and cranberry juice in 30 minutes at about 450 MPa (4,350 atmospheres, 64,000psi) but not at about 300 MPa (3,000 atmospheres, 43,000 psi).
The reason for very limited success in preservation of foods by pressure alone is the resistance of spores of many strains of bacteria to great pressure. For example 10% of spores of Bacillus cereus survived pressure of 900 MPa (8,700 atmospheres 128,000 psi)(Shigehisa, et al, 1991). Spores of Bacillus coagulans are reduced by 90% at 450 MPa (4,350 atmospheres, 64,000 psi) for 30 minutes, but higher pressure had even less killing effect (Gould and Sale, 1970). Spores of Bacillus cereus and Bacillus polymyxa survive at 1% to 2% after ambient temperature pressurization to 300 MPa (3,000 atmospheres, 43,000 psi) for 1 hour while 100% of Bacillus subtilis A spores survive under these conditions (Gould and Sale, 1970).
Bacteria can be destroyed if exposed to gas under pressure when the pressure is released and the gas expands. In this procedure nitrogen gas is mixed with bacteria so that under pressure the nitrogen dissolves with the bacteria so that at the time of rapid release the nitrogen forms a gas within the bacteria causing them to break apart. U.S. Pat. No. 1,711,097 to Kratzer illustrates such a method.
U.S. Pat. No. 5,316,745 to Ting and Raghavan discloses a method without examples of sterilization with pressure at very high ranges from 40,000 to 55,000 psi. The inventor states that "a series of pressurizations has been found to be more effective in killing or disabling the organisms" and teach that the pressurization cycles be repeated until sterilization is complete. No minimum time for, holding at high pressure is taught. The method claim includes a step "c" for exporting and importing material to be sterilized.
U.S. Pat. No. 5,288,462 to Carter and Brazell discloses a method for sterilizing materials which involves very rapid decompression, from 1 to 10 milliseconds. This method apparently works at pressures as low as 1,000 psi. The method was applied to dental instrument sterilization and the effect of rapid decompression on food appearance and quality is not known.
U.S. Pat. No. 5,228,394 to Kanda, et al, describes but does not claim a three chamber system method in which food liquids such as fruit juice, milk, and sake are pressurized to a pressure of 2,000 to 4,000 atmospheres (39,400 psi to 58,800 psi) for 5 to 20 minutes.
U.S. Pat. No. 5,328,703 is for a method of preserving juice using pressure of greater than 2,000 atmospheres with added proteolytic enzyme to assist in preventing separation of components. Claim 6 distinctly sets a time between 5 and 120 minutes. This method only claims fruit juice.
Japanese patent JP 4304838 A describes a process using pressure greater that 1000 atmospheres followed by heat treatment of greater that 60 degrees C. Heating was necessary in this process as in many others.
Because of such reports and experience no general method has been developed for the use of hydrostatic pressure to preserve foods.